The present invention relates to surge protection systems for operation in gas turbine power plants and more particularly to such systems for use in combined cycle electric power plants.
In combined cycle and other gas turbine electric power plants, which employ an axial flow compressor it is desirable to provide reliable protection against compressor surge since compressor operation under abnormal surge conditions can result in substantial equipment damage and substantial power generation downtime. Surge protection is provided in the first instance through proper compressor and turbine apparatus design so that surge conditions are not reached during normal operation.
Surge occurs when the compressor operates along its discharge pressure-inlet flow characteristic to the point where the discharge pressure drops below the combustor shell pressure. The compressor outlet flow then goes to zero and tends to reverse its direction, but shortly thereafter the flow condition is violently cleared when the combustor shell pressure bleeds down below the compressor discharge pressure. The compressor then returns to operation on its discharge pressure-inlet flow characteristic curve and, unless the process conditions causing the instability have been removed, the compressor discharge pressure again drops below the combustor shell pressure and the surge cycle is repeated.
Surge operation is a stalling operation of the compressor and it can be likened to an operation resulting from the closing and opening of a check valve in the compressor flow path. The period of the surge cycle is a function of the compressor discharge volume. One common method for defining the compressor surge line is to plot pressure ratio versus compressor inlet volume flow. On that plot, one surge point exists for every compressor speed. In gas turbine plants, the delivery pressure is the combustor shell pressure.
To prevent surge during startup, bleed valves are typically used to increase compressor inlet flow volume. Inlet guide vanes can also be positioned, especialy during startup, to control compressor inlet flow volume and thereby avoid surge.
Surge limit control loops have also been employed to limit fuel demand and thereby prevent overfiring which would result in surge. For example, in the above referenced patent application, Ser. No. 319,114, surge limit control is implemented with a digital computer as a function of combustor shell pressure. However, known surge protection systems have been limited in the reliability of protection provided over the wide range of speed and pressure involved in turbine operation from ignition to maximum load.
The description of prior art herein is made on good faith and no representation is made that any prior art considered is the best pertaining prior art nor that the interpretation placed on it unrebuttable.